1. Technical Field
The invention relates to the field of hydrophones and, in particular, to a hydrophone comprising a flat piezoelectric base element which holds a linear arrangement of detector elements having piezoelectrically active areas to which are attached on both sides respectively a pattern of metalic electrodes and connecting electrical conductors, the electrically active area of the base element being polarized in the direction of the thickness of the foil and having only a small localized area in planar contact with the base element.
2. Description of the Prior Art
Miniature hydrophones are known which serve for the measurement of ultrasonic fields within a sound conducting medium. For a base element one may employ a membrane of polyvinylidene fluoride PVDF or co-polymers of vinylidene fluoride with tetrafluorethylene or with trifluorethylene with a thickness, for example, of 25 .mu.m, which is stretched as a membrane between two rings of metal. The piezoelectrically active area is located at its mid-point which is provided with electrodes and this forms a detector element. The foil can furthermore be stretched at the end of a coaxial cable and its piezoelectrically active area, with a spread of about 1 mm, arranged at the point of a needle. With this needle design, one obtains a sensitivity of about 6.3.times.10.sup.-8 V/Pa ("Miniature Piezoelectric Polymer Ultrasonic Hydrophone Probes", P.A. Lewis, Ultrasonics, September 1981, pps. 213 to 216). The detection of ultrasonic fields with greater range with this type of detection containing but a single detector element, however, requires the expenditure of relatively long times.
An attempt has been made to provide foils of polyvinylidene fluoride PVDF. These foils have a linear arrangement of a large number of piezoelectrically active areas, each area being provided with a pattern of electrodes and connecting conductors generally made of metal. These areas are formed on both sides of the foil by evaporation. The detector elements have a diameter of about 2 mm and their capacity is about 8 pf. An amplifier is located at the end of a 30 cm long cable. With this arrangement one obtains a sensitivity of about 3.2.times.10.sup.-6 V/Pa. (J. Accoust. Soc. Am., Vol. 61, Suppl. No. 1, Spring 1977, Pages 17 and 18). However, it is desirable to surpass this level of sensitivity.